In forested catchments the hydrologic cycle, involving precipitation, interception, evapo-transpiration, overland ?ow, subsurface ?ow, groundwater ?ow, and stream ?ow is closely linked to water quality in that water movement through the forest ecosystem also transports sediment, and dissolved nutrients, as well as fertilizers, and pesticides if they are present. Understanding relationships between forested ecosystems and quality of surface and subsurface water associated with these systems is a key component of sustainable forest management because changes in water quality may result from forest management practices. These changes can re?ect either positive or negative outcomes of forest practices. For example, logging road construction and harvesting of timber with improper consideration for erosion control can cause increased sedimentation of stream water and a degradation of water quality. In contrast, conversion from agricultural crop production to forestland can improve water quality by decreasing erosion rates and creating long-term storage pools (e.g., forest ?oor, woody biomass) for carbon and nutrient retention. This article provides a synthesis of our current thinking regarding (1) the concept of water quality, (2) the role of forested watersheds in providing water of relatively high quality, and (3) commonly evaluated water quality parameters and potential effects of forest practices on these parameters. The primary focus is on the relationship between water quality characteristics of streams draining forested water-sheds and forestry practices.

[...] Clearing of riparian vegetation is the primary forest management practice that can cause elevated stream temperature, particularly in small headwater catchments. This is the result of increased stream exposure to direct solar radiation. Temperature increases of as much as 151C have been observed in forest streams when riparian vegetation has been removed. However, the magnitude of the response is tempered by stream discharge, streambed characteristics, channel morphology, stream surface area, and degree of hyporheic exchange and groundwater in?ux along the stream length. [...]

[...] Nevertheless, excessive suspended sediment loads in streams are the major water quality concern for forest management because poorly planned forest management activities on hillslopes or in the vicinity of the stream channel that cause erosion can add to naturally derived levels of suspended sediment. Increases in suspended sediment levels resulting from erosion and soil mass movement (i.e., landslides) can degrade drinking water quality, detract from recreational values, decrease stream depth, ?ll pools in the stream channel, increase stream width, and cause sedimentation of gravel beds which lowers their permeability and degrades their habitat quality for spawning ?sh. [...]

[...] Pesticides Applications of pesticides to forest lands are just a fraction of those applied to agricultural lands and pesticide concentrations associated with forest management practices are generally many times less than those used on agricultural lands. However, there are circumstances where forestry applications can cause degradation of water quality and potential impacts on stream biota. Pesticides, including herbicides for vegetation control and insecticides for control of damaging insects, are often used for intensive forest management. Herbicides are used to control competing vegetation during forest stand establishment. [...]